fix a bug when dt is None

[Jedore]

No description provided.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@af1e2d4). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master      #35   +/-   ##
=========================================
  Coverage          ?   72.29%           
=========================================
  Files             ?        9           
  Lines             ?      296           
  Branches          ?        0           
=========================================
  Hits              ?      214           
  Misses            ?       82           
  Partials          ?        0

Impacted Files	Coverage Δ
django_apscheduler/util.py	100% <100%> (ø)

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